Anxiety disorders are among the most common psychiatric disorders affecting ~20 million American people. Because current medications are effective for only 50~60% of patients and have certain side effects or problems with tolerance or dependence, exploring novel neurobiological mechanisms and therapeutic approaches for anxiety disorders is still an arduous task. Our long-term goal is to explore novel mechanisms by which innovative therapeutic strategies for anxiety disorders can be developed. Accumulating evidence demonstrates that elevation of vasopressin (also known as arginine vasopressin, AVP; antidiuretic hormone) system facilitates anxiety via activation of V1a receptors (V1aRs). However, the mechanisms whereby activation of V1aRs increases anxiety have not been determined. The objective of this proposal is to determine the cellular and molecular mechanisms whereby V1aR activation facilitates anxiety. Our rationale is that determining the mechanisms whereby V1aR activation augments anxiety would stimulate the development and use of V1aR antagonists for the treatment of anxiety. Because elevation in glutamatergic functions underlies the generation of anxiety, we are testing the central hypothesis that activation of V1aRs facilitates anxiety by increasing the glutamatergic functions. The formation of the hypothesis is also based on our preliminary results demonstrating that activation of V1aRs facilitates the excitability of principal neurons and glutamatergic transmission in the ventral hippocampus which is closely involved in anxiety-like responses. We further showed that microinjection of AVP into the ventral hippocampus or optogenetically stimulating endogenous AVP release induces anxiogenic effects assessed by Elevated-Plus Maze (EPM) and Open Field Test (OFT). Aim 1 will identify the mechanisms underlying AVP-induced excitation of ventral hippocampal principal neurons. We will test the hypothesis that V1aR activation increases neuronal excitability via activation of TRPC4/5 channels by using knockout (KO) mice and optogenetic approach. Aim 2 will define the mechanisms whereby AVP facilitates glutamate release at the ventral hippocampal synapses. We will test the hypothesis that V1aR activation increases the release probability, quantal size and/or the number of release site or multivesicular release via interaction with TRPC4/5 channels, calcium/calmodulin-dependent kinase II (CaMKII) and synapsin I. Aim 3 will elucidate the mechanisms by which V1aR activation induces anxiogenic effects. We will test the hypothesis that TRPC4/5 channels, CaMKII and synapsin I are involved in V1aR- mediated anxiogenic effects using EPM and OFT. We believe that determining the mechanisms underlying V1aR-mediated increases in anxiety would provide a novel approach for anxiety therapy.